1. Field of the Invention
The present invention relates to an improvement to an electron cyclotron resonance (ECR) ion source in particular permitting the production of multicharged ions.
It has numerous applications as a function of the different values of the kinetic energy of the ions produced, in the field of ion implantation, microetching and more particularly in particle accelerator equipment used both in the scientific and medical fields.
2. Description of the Related Art
In electron cyclotron resonance ion sources, the ions are obtained by the ionization in a sealed enclosure, such as a superhigh frequency cavity, of a gaseous medium constituted by one or more gases or metal vapours by means of electrons highly accelerated by electron cyclotron resonance. This resonance is obtained as a result of the combined action of a high frequency electromagnetic field injected into the enclosure containing the gas to be ionized and a magnetic field prevailing in the same enclosure and whose amplitude B satisfies the following ECR condition B=F.multidot.2.pi.m/e, in which e represents the electron charge, m is mass and F the frequency of the electromagnetic field.
In these sources, the ion quantity which can be produced results from the competition between two processes, on the one hand the formation of ions by electron impact on neutral atoms constituting the gas to be ionized and on the other the destruction of the same ions by single or multiple recombination during a collision of the latter with a neutral atom. This neutral atom can come from a gas which has not yet been ionized or can be produced on the enclosure walls by the impact of an ion on said walls.
This disadvantage is obviated by confining, within the enclosure constituting the source, the ions formed, as well as the electrons used for their ionization. This is brought about by creating within the enclosure radial and axial magnetic waves defining a so-called "equimagnetic" surface, having no contact with the enclosure walls and on which the electron cyclotron resonance condition is satisfied. This surface is shaped like a rugby ball. The closer said equimagnetic surface is to the enclosure walls, the greater its efficiency, because it permits the limitation of the presence volume of neutral atoms and therefore the quantity of collisions between neutral atoms and ions. This surface also makes it possible to confine the ions and electrons produced by ionization of the gas. As a result of this confinement, the electrons created have the time to bombard several times the same ion and completely ionize it.
Such an ion source is described in the document filed on Mar. 13, 1989 in the name of the present Applicant and which was published under no. FR-A-2 595 868.
FIG. 1 diagrammatically shows a prior art ion source. Said source comprises an enclosure 1 constituting a resonant cavity which can be excited by a high frequency (HF) electromagnetic field. This electromagnetic field is produced by an electromagnetic wave generator 3 and is introduced into the enclosure 1 by means of a waveguide 5 and a transition cavity 20. This source also comprises an externally shielded magnetic structure 7, 9, 11, whose shield 11 makes it possible to only magnetize the volume in the enclosure 1 which is useful for ECR.
Apart from the shield 11, said magnetic structure also comprises permament magnet 7 and solenoids 9 arranged around the enclosure 1 and respectively creating a radial magnetic field and an axial magnetic field. These two magnetic fields are superimposed and distributed throughout the enclosure. Therefore they form a resultant magnetic field, which defines the resonant equimagnetic surface 13 within the enclosure 1.
A magnetic axis 15, which is also the longitudinal axis of the source, traverses the shield 11 via two openings 17 and 19 made in said shield 11 to respectively permit the extraction of ions from the enclosure 1, as well as the introduction of electromagnetic waves and gaseous or solid samples.
A first and a second ducts 21, 23 connect the opening 19 of the shield 11 to the respective openings 25 and 27 of the transition cavity 20, said openings being located on the side faces of the cavity 20, which is shaped like a cube.
The ratio of the diameters of these two ducts 21, 23 is such that it is possible to liken the latter to a coaxial line having a characteristic impedance of approximately 85 ohms. Such a coaxial line preferably propagates a transverse electromagnetic (TEM) mode, in which the electromagnetic field E is transverse to the propagation direction of the waves and perpendicular to the surface of the conductors, i.e. The ducts 21, 23.
In order to ionize a gas, the latter is introduced into the enclosure i by means of a gas duct 30 connected to the opening 27 of the transition cavity 20. The gas and the electromagnetic waves introduced into the cavity 20 are transmitted to the enclosure 1 by first and second ducts 21, 23, whose function is to make it possible to transmit said waves to said enclosure and inject them along the longitudinal axis 15.
It is also possible to create ions from a solid sample introduced in the form of a rod into the duct 23. However, throughout the following description, the ionization of a gas will be used as an example.
In the enclosure 1, the combination of the axial magnetic field and the electromagnetic field makes it possible to strongly ionize the gas introduced. The electrons produced are then highly accelerated by electron cyclotron resonance, which leads to the formation of a hot electron plasma confined in the volume defined by the equimagnetic surface 13.
The ions then formed in the enclosure I are extracted therefrom by an electric extraction field generated by a potential difference applied between an electrode 31 and the enclosure 1. The electrode 31 and the enclosure 1 are both connected to an electric power supply 33, the electrode 31 being positioned outside the opening 17 of the enclosure 1.
In order to check the intensity of the ion stream, it is possible to check the average power of the electromagnetic field by acting on a pulse generator 35, which is positioned upstream of a power supply 37 connected to the electromagnetic wave generator. The pulse generator 35 controls the said power supply 37 by adjusting the useful cycle, namely the ratio between the duration of a pulse and the period of the pulses.
Moreover, total pressure measuring means 39 are connected to an input of a comparator 41, whose output is connected to a valve 43 of the gas duct 30. To a second input of the comparator 41 is applied a reference voltage R and is compared with the measured value of the ion stream in order to give, at the comparator output, the value to be transmitted to the valve 43. This valve 43 makes it possible to act on the gas quantity to be introduced into the enclosure 1, so as to automatically regulate the ion stream.
Moreover, an adaptation piston 45 connected to a third lateral opening 29 of the cavity 20 makes it possible to regulate the internal volume of said cavity 20. The regulation of the piston 45 is used for tuning all the internal volumes of the cavity 20 to the frequency of the electromagnetic waves in order to obtain a minimum of reflected waves, i.e. waves returning to the wave generator 3. When these internal volumes are tuned to the frequency of the electromagnetic waves, the waves injected into the cavity 20 by the generator 3 are almost entirely transmitted by the ducts 21 and 23 to the plasma-containing enclosure I and are then absorbed by the equimagnetic surface 13.
In said prior art ion source, the second duct 23 is transparent to the electromagnetic waves at its end 23a, which is close to the opening 19 of the enclosure 1 positioned facing the shield 11.
In the internal volume of said transparent part 23a there is an axial magnetic field from the solenoids, an electromagnetic field and a high gas pressure. The electromagnetic field results from the electromagnetic waves transmitted between the first duct 21 and a non-transparent part 23b of the second duct 23 and which traverse the transparent part 23a of the second duct 23. Therefore, an electron cyclotron resonance can take place in the interior of the end 23a of the second duct 23 in a volume where there is a high gas pressure.
This end transparent to the electromagnetic waves consequently constitutes a self-regulated preionization stage, where the excess incident power of the electromagnetic waves is transmitted, without reflection, to the ECR zone constituted by the equimagnetic surface 13.
Thus, the more dense the plasma produced by electron cyclotron resonance (or preionized plasma) within the duct end 23a, the better the transmission of the electromagnetic waves, whereby said preionized plasma becomes conductive. More specifically, the preionized plasma is raised to a potential imposed on it by the immediate presence of the-conductive part 23b of the duct 23, which is itself exposed to the voltage of the power supply 33 via the duct 21 and the enclosure 1.
The plasma confined within the equimagnetic surface 13 is naturally raised to a positive potential compared with the enclosure 1. Thus, the electrons of said confined plasma are heated by cyclotron resonance of the electrons and certain of the latter which are of too high energy escape from the confinement. They will then strike against the enclosure 1 which, under this action, is negatively charged. Therefore the confined plasma has a more positive polarity than that of the enclosure.
In addition, the potential difference created between the enclosure 1 and the confined plasma is the cause of an electrical field E. The latter permits the transfer of confined ions to the opening 17 of the enclosure 1.
However, the preionization plasma extending up to the equimagnetic surface 13 is in contact with the confined plasma. However, said preionization plasma is conductive and is raised to the same potential as the enclosure 1. The electrical field E is then disturbed, which affects the capacities of the ion source.
The removal of the conductive part 23b of the second duct, whilst increasing the transparent part 23a would effectively permit the isolation of the preionization plasma from the confined plasma. However, in such an apparatus, the transmission of the electromagnetic wave from the generator 3 is no longer ensured, because said transparent part 23a is no longer conductive. However, the wave requires two coaxial conductors forming a coaxial transmission line in order to be transmitted.